1. Field of the Invention
The present invention relates to an optical disc device for recording and/or reproducing information by using laser beams, which is particularly suitable for adjusting a set value of laser power.
2. Description of the Related Art
Recently, various optical recording media such as a compact disc (CD) and a digital versatile disc (DVD) have been commercialized and are widely available. With respect to the optical recording media, in general, laser power is optimized on a system side before recording/reproducing operations because optimum laser power is changed according to a medium.
Of the optical recording media, with respect to a CD-RW (rewritable) and a DVD−RW (rewritable), a recording laser power adjusting method (γ method) using a γ value is described in each corresponding standard specification. According to the adjusting method, recording laser power is set to an optimum value based on a modulation factor of a reflection light intensity detected in trial writing and a target γ value set in advance for a corresponding medium.
With respect to a DVD+RW, the use of γ methods, particularly, a linear fit method out of the methods is recommended in its specification. According to the linear fit method, product values Sn (=m(Pwn)×Pwn) are calculated based on several kinds of recording laser powers Pwn set in trail writing and modulation factors m(Pwn) of reflection light intensities detected in trial writing at the respective recording laser powers Pwn. And a relationship characteristic between the calculated product values Sn and the recording laser powers Pwn is subjected to linear approximation to obtain laser power Pwth when an approximate line (Sn) becomes zero. Then, optimum laser power Pw (=Pwth×(1+1/γ)) is calculated based on the obtained laser power Pwth and a target γ value (γ) for the medium.
A laser power setting method based on the γ method or the linear fit method is described in, for example, Jan. 22, 2002, Triceps Corporation publication, “White Series No. 218 DVD+RW/R”, pp.111-113 (4.2 OPC (Optimum Write Power Control) method).
When a system state (medium temperature, semiconductor laser temperature etc.) changes after the setting of the recording laser power as described above, for example, the recording sensitivity of the medium is changed from an initial state. With this change, the recording laser power initially set is gradually deviated from an adequate value. In order to solve such a problem, on the system side, the initially set recording laser power is generally adjusted as appropriate according to a current system state.
For example, as the adjusting method, it is possible to use a method of monitoring a reflection light intensity during recording operation (reflection light intensity from formed recording mark) based on a signal level from a photo detector (optical pickup) and adjusting recording laser power such that the monitored reflection light intensity becomes an initially set reflection light intensity. However, in this case, it is necessary to separately perform power adjustment processing in parallel with the recording operation. Therefore, a problem in that a processing sequence during the recording operation is complicated occurs. In addition, the adjustment processing is not processing for monitoring a modulation factor of disc reflection light. Therefore, a formation state of the recording mark cannot be accurately detected, with the result that the method is disadvantageous in that the recording laser power cannot be adjusted with high precision.